The dayside cusp is a region of Earth's magnetosphere. Relatively low energy (in the 100s of electron-volts range) solar wind electrons and protons that enter the magnetosphere in the dayside cusp regions near the magnetic poles can impact Earth's upper atmosphere. Also, Earth's ionosphere at high latitudes expands supersonically into the dayside cusp, causing charged particles from the atmosphere to mix with the hot, tenuous, solar wind plasma. Image courtesy the COMET and HAO programs at UCAR/NCAR.